Presume you're using an insulated "break" gland at the shed end and the shed is forming a TT system? The RCD operating residual current will be low so the main consideration will be physical resilience hence affected by mechanical protection arrangements. It used to be tabulated as MINIMUM 2.5mm2 if protected or 4.0mm2 if not protected but other conditions apply if buried in soil/insulated/bare/copper/steel If you haven't got a copy of the current wiring regulations or "on site guide" try the local library.